Where there are radio-opaque areas where communication is required, a separate antenna will be used in each. Each antenna will be connected either to a separate radio for that particular coverage space, or to a transmission line which is routed to a radio in a central location. At that central location, the transmission line can be summed or divided (for receiving or transmission, respectively) to a common radio, or switched among other similar antennas to a single radio. In either event, it is not possible to simultaneously communicate with any antenna in the system, and there is a great deal of expensive transmission line required (and which must be installed throughout the building) to connect each antenna separately, and a means of summing or switching the antennas among one or more radios is required.
Prior art has attempted to maintain communications in such scenarios by the use of “leaky feeder” transmission line, which is a coaxial cable into which radiating slots have been cut to cause the transmission line to function as a series of slotted antennas. Unfortunately, this has several drawbacks. The pattern is controlled by the rotational position of the cable, which is difficult to control during installation and use. The antenna gain is poor, meaning that it is inefficient in receiving and transmitting. The slot can generally be tuned to only one narrow frequency, making it limited in coverage for a given cable. The diameter of the cable has to be relatively large to support its use as a VHF or UHF slot antenna, according to the physics of the antenna design. Thus leaky feeders have not enjoyed wide use in solving the multi-antenna coverage problem.
This disclosure teaches a solution for overcoming the problems of the leaky feeder approach and provides multi-antenna coverage of an area in an efficient and affordably manner.